The present invention relates to a liquid crystal display device used for a laptop, a notebook personal computer, a portable equipment or the like.
In a conventional COG (Chip On Glass) mounting structure of a simple matrix liquid crystal display device, as shown in FIG. 7, a plurality of transparent electrodes for switching liquid crystals and formed on transparent substrates 1a and 1b are comprised of a pixel portion 2, a drawn portion 3, and a connection terminal portion for connecting a slim-shaped liquid crystal driving driver IC 4.
The drawn portion 3 is a portion where connection terminals for connecting the liquid crystal driver IC 4 are connected to pixels constituting a display screen. The configuration of the drawn portion is as follows depending on the positional relationship between the connection terminals and the pixels.
That is, one or two transparent electrodes at the center of the connection terminal portion have the drawn portion having such a configuration that the transparent electrodes are linearly connected from the connection terminal portion to the pixel portion without being bent, and a plurality of transparent electrodes at right and left sides of the center terminal have such a configuration that they are linearly drawn from the connection terminal portion, and are connected to the pixel portion after being bent once. Thus, the drawn portion is formed of the linearly drawn area and the bent and drawn area, and the plurality of transparent electrodes are formed in parallel with each other at the same pitch for the respective areas.
In a conventional design using a TCP (Tape Carrier Package), since the pitch of the TCP could be designed at a value close to a pixel pitch, the distance between a pixel and a connection terminal including a sealing area for bonding two transparent substrates and a non-display area was 4.5 mm.
However, in the case where the connection terminals formed at the connection terminal portion have a pitch of not larger than 80 microns, the number thereof is not less than 160, and a pixel pitch of a liquid crystal display device is not less than 240 microns, if the distance between the pixel and the connection terminal is not made more than 4.5 mm, the width of the transparent electrode in the drawn portion becomes thinner than a connection pitch of the liquid crystal driver IC. However, increasing the size of the drawn portion does not meet the market needs. Further, as the output terminal number of one IC increases from 160 terminals to 240 terminals, the width of the transparent electrodes in the drawn portion becomes thin. Thus, patterning inferiority of the transparent electrode such as disconnection is increased so that the yield is lowered. Also, the picture quality of a liquid crystal display device is affected by the dielectric constant of the liquid crystal, frame frequency, display area per line, output resistance value of the liquid crystal driver IC, resistance value of the transparent electrode, and connection resistance value when both are connected to each other, so that there arises such a problem as shadowing(picture trailing) or contrast deteriorated.
Since the transparent electrode is made of a material of an oxide of IN and Sn, the resistance value thereof is high, the sheet resistance value is 10 .OMEGA./.quadrature. at a mass production level, and lowering of the resistance of the transparent electrode is limited. Thus, the resistance value of the transparent electrode in the drawn portion from the connection terminal portion for the liquid crystal driver IC to the pixel portion, becomes high. Especially in the liquid crystal display device having a COG mounted slim-shaped liquid crystal driver IC, the transparent electrode drawn from electrode at the end of the liquid crystal driver IC becomes long, so that the screen becomes inferior in contrast. If an auxiliary electrode is formed of a metal film by plating, sputtering or the like in order to lower the resistance value of the transparent electrode, the cost is increased so that the method does not satisfy the request from the standpoint of the market price and has no merit. Even if an auxiliary electrode is formed by a method such as electroless plating on an exposed transparent electrode after fabrication of a liquid crystal cell, since the transparent electrode on which the auxiliary electrode is formed must be exposed, the width of the drawn portion must be made large, which eventually results in the increase of a frame portion of a liquid crystal panel.